Gas turbines must comply with ever increasing regulatory emissions standards while maintaining or optimizing performance and cost. Mandated emissions regulations to reduce oxides of nitrogen (NOx) are the main driving force behind the development of new and costly technologies involving premixed combustion, catalytic systems, and post combustion clean-up. To comply with emission standards, lean-burning catalytic systems have advanced significantly to provide a stable (dynamic-free) burn with low NOx emissions. However, such systems require preheating as the catalyst will not light at all combustor inlet temperatures. In aeroderivative and advanced cycle engines, the temperature of the combustor inlet at full power conditions (at or above 800° F.) may exceed catalyst light-off temperatures (about 700° F.). But even for these systems, the catalyst will not be operational at part power conditions. To preheat the gas to necessary inlet temperatures, a pre-burner is located upstream of the catalyst. This preburner can be the primary source of NOx emissions for lean-burn catalytic systems.
Furthermore, the use of lean-burn catalytic systems in advanced cycle engines that require high firing temperatures (above 2200° F.) is limited. In such cases, high flame speeds and temperatures lead to overheating of the catalyst bed and reduced product life.
Fuel-rich catalytic systems do not face the noted light-off problem as the fuel-rich catalysts are much more active at lower temperatures. However, the products of the fuel-rich reaction system must then be burned in a more conventional homogeneous environment, i.e., normal combustion, which produces NOx emissions. While overall NOx emissions with such systems are low relative to those within a more conventional gas turbine (e.g., 9-15 ppm), they still are on the order of 3 ppm. In some installations, post-combustion clean-up will still be required should local regulations mandate emissions levels below 3 ppm. Variation in combustor hardware, construction, location, and ambient conditions suggest that the 3 ppm emissions may not be attainable easily for the next generation of burners.
Therefore, there is a need for a system that resolves conflicts with the high light-off temperatures and the limitations to the high firing temperatures associated with use of the lean catalytic burners to provide near 1 ppm emissions without any drawbacks in performance or cost. Additionally, there is a need for a method for combustion of conventional fuels such as methane, natural gas, propane, home heating fuels, diesel fuel, and standard aircraft fuels in such a manner as to limit the formation of pollutants such as nitrogen oxides (NO+NO2, together known as NOx).